In continuous ink jet printers of the binary kind, a print head directs a plurality of electrically conductive ink drop streams, past respective charge electrodes, toward a print zone. The charge electrodes are located opposite the drop's break off region and are selectively energized or non energized to a predetermined voltage level (in accord with information signals) to induce an opposite polarity charge, or no charge, on the adjacent ink drops. The drops that are induced with a charge of opposite polarity (i.e. opposite the electrode voltage polarity) are deflected to a downstream, catcher portion of the print head. Non-charged drops pass onto the print medium.
If one or more of the ink jet streams becomes misdirected (e.g. by a dirt particle caught in its orifice). the electrically conductive ink can accumulate on the charge electrode surfaces and cause a conductive path between the electrode surface and an electrically grounded portion of the printer (often the orifice plate). The electrodes, their leads and the ink offer a low resistance path so that a significant "shorted circuit" current can develop quickly, and damage the electrodes (and in some instances the orifice plate).
One prior art approach for detecting such shorted circuit is illustrated in FIG. 1. Thus, a resistor 1 is placed in series with the high voltage supply 2 and the high voltage input terminal of the charge electrode driver circuitry 3. The output leads of the charge electrode driver circuit are connected to the charge electrode leads 4. These leads normally show a high resistance to ground potential, as they are molded into an epoxy substrate which has a very large resistivity. Therefore, the electrical current carried by the charge leads during nominal printing operations of a print head is negligible, typically much less than 1 microamp.
In the event a shorted circuit current flows through one or more of the charging leads, a voltage will develop across the resistor 1. This voltage is compared to a predetermined reference potential by a comparator circuit 5. If the voltage across resistor 1 is larger than a reference potential, the comparator circuit will output a signal enabling a sequence of system shut down instructions, e.g. including switching off of the high voltage power supply and ink jets. If the voltage across resistor 1 is less than the reference potential, then the printer is allowed to operate normally.
A major disadvantage of the FIG. 1 approach for ink jet short detection is the relatively large current (e.g. 0.5 to 1.0 milliamp) which must flow through the charging lead(s) in order to initiate a system shut down. Currents of this magnitude are capable of causing charge plate lead and/or orifice plate damage; however, such large threshold currents are necessary because of spurious electrical noise on the high voltage power supply line. If the comparator circuit is designed to be triggered by significantly smaller short circuit current, the noise signals can initiate an unwanted system shut down. When false shut downs become a chronic problem for the user, an unnecessary print head replacement may be made.
Another problem is that the FIG. 1 detection system can only be used at periodic test intervals, and not when printing information is being imparted to the charge electrodes. This prior art approach therefore requires a complicated hardware counting procedure for controlling, sampling and interpreting its short detection circuit. U.S. Pat. Nos. 4,171,527 and 4,439,776 disclose other examples of detection circuits which suffer similar disadvantages.
Commonly assigned U.S. patent application Ser. No. 265,102, filed Oct. 31, 1988, now U.S. Pat. No. 4,928,115 and entitled "Continuous Ink Jet Printer Having Remotely Operable Print Head Assembly" discloses a continuous ink jet printer device which, among other features, includes a catcher having a dielectric drop impact surface and an electrically conductive drop discharge portion. This configuration enables the electrical current, created by caught droplets, to be detected by an electrometer coupled to a discharge portion of the catcher. The electrometer is employed during start-up procedures to detect and adjust the phase relation between drop generation and the drop charge signal, e.g. in accord with the method of U.S. Pat. No. 4,616,234. The print head of the Ser. No. 265,102 application, now U.S. Pat. No. 4,928,115 utilized the short detection approach shown in FIG. 1.